1. Field of the Invention
This invention relates generally to optical devices, and more particularly to optical fiber direct view display devices.
2. Prior Art
Fiber optic display devices are known and are employed in a variety of ways including display screens in public places such as airports, shopping malls, and the like. In such systems a fiber optics conduit is used to enlarge an image obtained from a transparency, video display or other source and display it on a viewing surface that is substantially larger than the surface of the source image.
While the prior art systems do provide a way to enlarge and display an image on a viewing surface, such systems tend to be inefficient in that they transfer only a small proportion of the light from the image to the viewing screen. This occurs because, although the fiber optic medium is itself an efficient light transfer mechanism, inefficient coupling between the input and output faces of the fiber optics and the external environment substantially reduces the efficiency of the system. Examples of such prior art systems are illustrated in U.S. Pat. Nos. 4,116,739; 4,208,096 and 4,650,280. These prior art systems transmit approximately 12% to 34% of the energy imposed on their input faces to their output faces. All of these prior art systems also generate Lambertion fans at their output faces with spread angles of up to 180 degrees, thereby wasting a substantial portion of the energy that is transmitted from the input face.
Among the reasons for the lack of efficiency is the lack of perpendicularity of the viewing face to the axis of the optical fibers. This causes a portion of the light exiting the fiber to be reflected back toward the source, a phenomena known as critical trapping. Trapping occurs when the internal bounce angle of a fiber plus the angle of tilt from the perpendicular of the viewing face exceeds the critical trapping angle. The internal bounce angle is approximately half of the inherent cone angle, or field of view, of the fiber. The critical trapping angle depends on the optical medium but is approximately 42.degree. for plastic fibers.
Additionally, any tilt of the object plane without an equal tilt of the image plane will introduce increasing anamorphic distortion into the regenerated image. Anamorphic distortion increases with the offset from perpendicularity between the image and object planes and the optical fibers, and results in an elongation of the regenerated image away from the fiber axis, the amount and direction of elongation corresponding to the tilt of the object plane.
Irregularities on the interfaces between air and the object planes can also reduce transmission efficiency. A minute scratch or coarse polish can trap rays as they leave the object plane. The input and output cone angles of the fibers also cause inefficiencies when they are not optimized for the imaging source and the audience window. These various factors combine in the prior art systems to reduce transmission efficiency to less than fifty percent of the energy impinging on the input face.